preparation

Make the cake:

Preheat the oven to 350°F. and butter and flour a 9 1/2-inch springform pan. In a bowl with an electric mixer cream together the butter, the sugar, and the vanilla until the mixture is light and fluffy and beat in the eggs, 1 at a time, beating well after each addition. Into another bowl sift together the flour, the baking powder, and the salt and beat the mixture into the butter mixture in batches alternately with the milk, beginning and ending with the flour mixture. Pour the batter into the prepared springform pan and bake the cake in the middle of the oven for 50 to 55 minutes, or until a tester comes out clean. Let the cake cool in the pan on a rack.

Make the custard:

In a saucepan whisk together the cornstarch, the sugar, and the milk, add the eggs, the cream, and the salt, and whisk the mixture until it is smooth. Scrape the seeds from the vanilla bean, reserving the pod for another use, add them to the cream mixture, and bring the custard to a boil over moderate heat, whisking constantly. Boil the custard, whisking, for 2 minutes, remove the pan from the heat, and whisk in the butter. Let the custard cool completely, whisking occasionally.

Make the glaze:

In a metal bowl set over a saucepan of barely simmering water melt the chocolate with the water, the butter, the corn syrup, and the salt, stirring until glaze is smooth, and remove the bowl from the pan.

Remove the cake from the pan, halve it horizontally with a long serrated knife, and arrange the bottom half, cut side up, on a plate. Top the bottom half with the custard, spreading the custard to the edge, put the remaining cake half, cut side down, on the custard, and pour the glaze on top of it, spreading the glaze to the edge and letting it drip down the side. The Boston cream pie may be made 1 day in advance and kept covered loosely and chilled. Garnish the pie with the fruit and the blossoms.